1. Field of the invention
The present invention relates to a laser generator used in a laser processing machine, and in particular to a flow control device which adjusts the flow of laser gases in a laser generator.
2. Description of the Prior Art
Generally, three types of laser generators are known for use in a laser processing machine. These are the high speed axial, biaxial transverse, and triaxial transverse types.
At the present time, with a laser processing machine which uses a high speed axial type of laser generator, the surface roughness of the processed workpiece is inferior in comparison with a workpiece processed in a laser processing machine using a biaxial transverse or triaxial transverse laser generator, because the stability of the output beam (mode stability) is poor.
Various factors are considered to cause this problem, but the main three causes are believed to be turbulence in the laser gases, rippling of the laser current, and rippling of the blower gas pressure. Specifically, the turbulence in the laser gas flow depends on the shape of the laser tube. In conventional laser tubes the injection port for the laser gas is a T-shaped tube so that the laser gas flow must make a right-angled turn and a fluctuation is produced in the flow pattern. It has been observed experimentally that this laser gas flow fluctuation changes over a short time span, and the electric discharge also fluctuates along with the laser gas flow.
In addition, the results of measurements of the degree of surface roughness of an actually processed workpiece have shown a ripple having the same period as the ripple in the electric power source for the laser, indicating that the ripple in the power source has an effect on the degree of roughness of the surface of the workpiece.
Further, the ripple in the blower gas pressure results from the fact that, with the high speed axial type of laser generator, a Rootes blower is generally used. This type of blower is used because the laser gas is flowing at a high speed inside a narrow laser tube so that a high head and high flow volume are required. Because the Rootes blower discharges exhaust gases four times for each rotation of the motor, the production of the ripple in the gas pressure is unavoidable. A change in the impedance of the gas is apparent from the variation of the gas pressure. This fact, coupled with the velocity of the feedback response speed of the power control and its contribution to the variation of the beam output power results in a roughness in the surface of the workpiece from the laser process.
A more detailed consideration of the laser gas flow turbulence stated as the first cause of the roughness of the surface of the workpiece indicates that when a fluctuation in the laser gas flow is produced it is only natural that the electric discharge will also fluctuate. As a result of this fluctuation, a change related to position and time is also produced in the light amplification medium within the laser gases. As a result, when the distribution of the intensity of the output beam is measured by a laser beam analyzer, it is clearly shown that within a short time span there is a positional change. Accordingly, there is a large variation in the intensity distribution in the direction in which the gases are blown up (Y-axis direction) from the lower part to the upper part of the laser tube. In the present state of the art, this causes considerable difficulty because it is not possible to reduce the fluctuations in the flow of the laser gases flowing in the laser tube.